mfd: add platform_device sharing support for mfd

This adds functions to enable platform_device sharing for mfd clients.

Each platform driver (mfd client) that wants to share an mfd_cell's
platform_device uses the mfd_shared_platform_driver_{un,}register()
functions instead of platform_driver_{un,}register().  Along with
registering the platform driver, these also register a new platform
device with the same characteristics as the original cell, but a different
name.  Given an mfd_cell with the name "foo", drivers that want to
share access to its resources can call mfd_shared_platform_driver_register
with platform drivers named (for example) "bar" and "baz".  This
will register two platform devices and drivers named "bar" and "baz"
that share the same cell as the platform device "foo".  The drivers
can then call "foo" cell's enable hooks (or mfd_shared_cell_enable)
to enable resources, and obtain platform resources as they normally
would.

This deals with platform handling only; mfd driver-specific details,
hardware handling, refcounting, etc are all dealt with separately.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>